SAN FRANCISCO — For all its power, Verdi's "Don Carlos" is a problem opera. The composer, always unsatisfied and always flexible, tinkered with it at various stages of his career, adding and subtracting episodes, drastically altering the dramatic focus, reshaping the musical design.

For decades, most companies and audiences have accepted and expected a relatively trim four-act version that adheres to a comfortable if lazy Italian tradition and resorts to a mellifluous if lazy Italian translation.

Purists, on the other hand, have long preferred the five-act, French-language edition that Verdi prepared in 1867 for the world premiere in Paris. This edition tells us far more about the quasi-historical characters and their psychological dilemmas. The text fits the vocal lines with special suavity and point. Most important, the usually neglected pages contain music of grandeur and brooding eloquence.

Unfortunately, the so-called original version leaves some questions unanswered too. Scholars have recently discovered a number of extraordinarily effective passages that Verdi either cut or condensed prior to the opening--for practical rather than aesthetic reasons. Since no definitive version of "Don Carlos" exists, modern interpreters must face vexing choices.

The new production at the San Francisco Opera, conducted by John Pritchard and directed by John Cox, makes bold decisions, and settles, in the process, for a few disappointing compromises.

The language is French, or a reasonable operatic facsimile thereof. Good.

The structure adheres to the original five-act plan, which means West Coast audiences are seeing the crucial Fontainebleau scene for the first time. Excellent.

The edition chosen incorporates some of the music Verdi himself had suppressed, most notably an exquisite duet between the Queen and Eboli in Philippe's study and a magnificent ensemble after Rodrigue's murder. The latter increases sympathy for the King and introduces the poignant melody to which Verdi returned in the Lacrimosa of his Requiem Mass. Wonderful.

Despite the attentive research, however, the San Francisco forces have made some odd sacrifices. Big chunks of assertive music have been removed, for instance, from the otherwise reflective duets between Carlos and the Queen in both the first and last acts. At the other extreme, the editors have in at least one instance combined two different versions of the same scene. The composer clearly would have exercised a choice between the alternatives. Even in Verdi, there can be too much of a good thing.

One may quibble about the absence of this and the presence of that. One may worry about possible sprawl in a 4 1/2-hour opera-house marathon, and one may question the unbalanced placement of the two intermissions. Nevertheless, one can only applaud the probing seriousness of this venture, not to mention the sheer scope.

It would, no doubt, have been easy to mount a pretty, literal, conventional production to accommodate the musical adventure. Luckily, Terence McEwen resisted that popular temptation.

The company director allowed Cox to stage the opera in a vital, disturbing modern idiom. This, after all, is a play of dreams and moods as much as it is a spectacle.

Occasionally, Cox and Lazaridis may overstep the bounds of logic. After a while, one sees the limits of abstraction and stylization. Still, it will be impossible to forget their central image: a shadowy stage enclosed by movable tiers of statues--hooded figures that could represent saints or martyrs, oppressive demons of the Inquisition or its victims.

The cast, though generally undistinguished, was dominated Saturday night by two artists of extraordinary stature.

Robert Lloyd defined the anguished dignity of Philippe II with tense restraint, sang the noble music with granitic force, abiding intelligence and heroic pathos. As the idealistic Rodrigue, Alan Titus suggested that he has become a Verdi baritone with few peers; he traced the arching legato line with unflagging ease, articulated the text with rare elegance, rose gloriously to the mighty climaxes.

Neil Shicoff was eminently sympathetic in the title role, and he sounded compellingly febrile when he didn't force his lovely, essentially lyric tone off pitch in the high climaxes.

The others proved less satisfactory. Pilar Lorengar, way past her prime, offered wobbly, bleached-soprano sounds as Elisabeth de Valois. Stefania Toczyska reduced Eboli to a brazen-hussy cliche and sang with two voices: a nice, rich one for the low music and a tight, edgy one for the upper half of the role.

Joseph Rouleau, the veteran Canadian basso, made the ancient Inquisitor sound even older than he should. Philip Skinner's plangent singing of the ghostly Monk was compromised by amplification.

Pritchard conducted with propulsive affect when forceful personalities such as Lloyd and Titus held the stage. When the pallid routiniers took over, however, he buried his head in the score and merely beat time. This is no way for a music director to behave.